The present invention related generally to measurement test probes and more particularly to a measurement probe having an internal alignment fixture.
Measurement instrument probes generally consist of a probing head having an electrically conductive tube with an insulated and coaxially disposed probing tip extending from one end and a transmission cable, such as coaxial cable, extending from the other end. Disposed within the electrically conductive tube is a substrate having electrical components mounted thereon for compensating the probe head and/or terminating the head in the characteristic impedance of the transmission cable. The electrically conductive tube and a portion of the transmission cable extending from the tube is surrounded by insulating material. The other end of the transmission cable is connected to a compensation box having electrical circuitry therein that compensates for aberrations in an acquires signal and a connector for connecting the measurement probe to a test instrument, such as an oscilloscope or the like.
Two type of electrical measurement probes using the above design are passive voltage probes and active FET probes. Passive voltage probes generally have a large value series resistor, shunted by a capacitor, in the probing head coupled via the transmission cable to the series connected resistor and shunting capacitor in the measurement instrument. The voltage divider network developed by the series connected resistors presents a high impedance to a circuit under test at low frequencies, such as 5 MHZ or lower. As the frequency of the test signal increases the impedance level of the passive probe decreases. Active FET probes includes active components, such as field effect transistors or other active devices, that results in higher input impedance without the loss of the signal, i.e. low input capacitance that is typically less than 0.4 pF to 2 pF and high input resistance values that are typically greater than 100 K ohms. Active probes also have greater bandwidth and linear dynamic range than passive probes as well as driving the transmission cable at its characteristic impedance of 50 ohms.
As probe head size decreases and the bandwidth requirement increase into the multi-gigahertz range, the accurate positioning of the probe head components become critical. Traditionally, assembly fixtures are used to hold the various elements of the probing head in position as the head is assembled. This includes holding and accurately positioning the substrate in the electrically conductive tube for soldering, glueing or the like. However, because of the decreased size of the probing head and its associated components, traditional assembly fixtures cannot be used for holding and securing the substrate in the electrically conductive tube.
What is needed is an assembly fixture that holds and accurately positions a substrate in an electrically conductive tube of a probing head of a electrical measurement instrument. The fixture should be electrically neutral so an not to add electrical aberration to the electrical characteristics of the probing head. The fixture should also be small enough to be incorporated as part of the probing head.